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The EssentialAction is the new simpler type replacing the old Action[A]. To understand EssentialAction we need to understand the Play 2 architecture.

The core of Play2 is really small, surrounded by a fair amount of useful APIs, services and structure to make Web Programming tasks easier.

Basically, Play2 is an API that abstractly have the following type:

RequestHeader -> Array[Byte] -> Result

The above computation takes the request header RequestHeader, then takes the request body as Array[Byte] and produces a Result.

Now this type presumes putting request body entirely into memory (or disk), even if you only want to compute a value out of it, or better forward it to a storage service like Amazon S3.

We rather want to receive request body chunks as a stream and be able to process them progressively if necessary.

What we need to change is the second arrow to make it receive its input in chunks and eventually produce a result. There is a type that does exactly this, it is called Iteratee and takes two type parameters.

Iteratee[E,R] is a type of arrow that will take its input in chunks of type E and eventually return R. For our API we need an Iteratee that takes chunks of Array[Byte] and eventually return a Result. So we slightly modify the type to be:

RequestHeader -> Iteratee[Array[Byte],Result]

For the first arrow, we are simply using the Function[From,To] which could be type aliased with =>:

RequestHeader => Iteratee[Array[Byte],Result]

Now if I define an infix type alias for Iteratee[E,R]:

type ==>[E,R] = Iteratee[E,R] then I can write the type in a funnier way:

RequestHeader => Array[Byte] ==> Result

And this should read as: Take the request headers, take chunks of Array[Byte] which represent the request body and eventually return a Result. This exactly how the EssentialAction type is defined:

The Result type, on the other hand, can be abstractly thought of as the response headers and the body of the response:

case class Result(headers: ResponseHeader, body:Array[Byte])

But, what if we want to send the response body progressively to the client without filling it entirely into memory. We need to improve our type. We need to replace the body type from an Array[Byte] to something that produces chunks of Array[Byte].

We already have a type for this and is called Enumerator[E] which means that it is capable of producing chunks of E, in our case Enumerator[Array[Byte]]:

Which reads as the following: Take the RequestHeader then take chunks of Array[Byte] and return a response. A response consists of ResponseHeaders and a body which is chunks of values convertible to Array[Byte] to be written to the socket represented in the Enumerator[E] type.